Before you begin choosing a size, style or model of heat pump, you need to consider their pros, cons and cost-effectiveness.

Positives

Warm, dry and comfortable

Heat pumps can provide a level of all-round comfort not easily obtained by plug-in electric heaters. They can quickly bring a room up to temperature and then maintain it.

Lower heating costs

If you install a heat pump and keep your home about as warm as you do now, you could save a considerable amount in heating costs. But some of our subscribers with heat pumps tell us they use their units to keep their homes warmer than before, so their heating bills haven't dropped by much.

No gas charge

If you install a gas heater, you'll have to pay a gas connection charge (often around $30 per month) all year round, for a heating appliance you use for a few months.

Cooling

A reverse-cycle heat pump is the only type of home heating system that can both heat and cool a room.

Dehumidifying

If you switch a heat pump into cooling mode, it will also dehumidify the air in your house. In heating mode, heat pumps warm the air but don't dehumidify. Some units have a special dehumidifying mode, but this is designed for humid tropical climates and is not suitable for New Zealand winter conditions.

Air filtering

Many modern heat pumps incorporate a washable filter unit that removes dust and particles from the air. This could be an important feature for people with asthma and allergies. The filters need regular cleaning to keep the unit working at maximum efficiency. Some have a deodorising function as well.

House value

A heat pump installation may also add to your home's resale value.

Negatives

Noise

Whirring fans can be very annoying. Fans run in both the interior and exterior units all the time they are switched on. The fan in the inside unit of a heat pump should produce little more than a low hum in low-speed mode, but the compressor plus fan of the outside unit can be quite noisy. Check the manufacturer's specifications. Also, consider the impact on neighbours if the outdoor unit must be mounted near their sleeping areas.

Our 2009 member survey found noise was more likely to be an issue with older heat pumps - 15 percent of those bought before 2004 made enough noise to be "mildly disturbing". This fell to 7 percent for models less than two years old.

Not so good in low temperatures

Extracting heat from outdoor air gets more difficult as the temperature drops. Sometimes, especially on frosty nights, exterior heat pump units freeze-up and have to stop working for several minutes while they defrost. If you live in a frosty area see What are your needs for more about this problem.

Fifteen percent of owners in our reliability survey said their heat pump performed poorly in very cold weather. This was more likely to be a problem with older models that weren't inverter heat pumps - 25 percent didn't cope well with very low temperatures. 

What type of system explains more about types of heat pump.

Draughts

Circulating air can cause draughts - which means you need to think about where to place the unit. You don't want one on the wall just above your favourite armchair.

How much can you save on winter heating costs?

The answer to our question is not as simple as you might think. Modern inverter-style heat pumps can adjust their power output to suit the heating requirements of the moment and are most efficient when working at part load.

Just where this efficiency "sweet spot" is to be found is difficult for us to test. Our testing procedure pushes the units on reverse-cycle heating mode towards their maximum output, where they are less efficient.

So if your installer makes sure the unit is large enough (or even a little too large) for your needs, you should get more heat per dollar of electricity than our test results suggest.

But - even if they're not always running at an optimum setting - all the models in our test will give three or more times the heat value of every dollar you spend on electricity.